1. Field of the Invention
This invention pertains in general to a method for manufacturing a semiconductor device and, more particularly, to a method for preventing native oxide growth during the formation of a silicon nitride layer.
2. Description of the Related Art
The presence of native oxide, or silicon dioxide (SiO2), in certain components of a semiconductor device may result in unintended electrical characteristics. For example, the presence of native oxides at capacitor nodes lowers the dielectric constant of the capacitor, which lowers the capacitance, and therefore results in abnormal resistance-capacitance electrical characteristics. Thus, in a conventional semiconductor manufacturing process, a silicon wafer is usually cleaned with a diluted hydrogen fluoride (HF) solution to remove native oxides.
However, undesired native oxides may reappear during subsequent manufacturing steps, such as during the formation of a silicon nitride (Si3N4) layer. For a manufacturing process in which layer of silicon nitride is grown, a silicon wafer is placed inside a cassette that holds a plurality silicon wafers. The cassette is placed inside an area of a furnace known as a xe2x80x9cload lock.xe2x80x9d To prevent native oxides from forming before the nitridation process, the load lock is filled with nitrogen gas to dilute the concentration of oxygen present in the load lock to suppress oxidation of the silicon wafer. After the load lock is filled with nitrogen gas, the cassette containing the wafers is transferred by a robotic arm into an apparatus known as a xe2x80x9cboat,xe2x80x9d and the boat is generally transferred upwardly into the furnace chamber. This is known as xe2x80x9cboat up.xe2x80x9d Again to prevent native oxides from forming, the furnace chamber is filled with nitrogen gas during boat up. After the boat is placed inside the furnace chamber, a vacuum is created inside the furnace chamber, a procedure known as xe2x80x9cpump down,xe2x80x9d after which ammonia gas (NH3) is supplied inside the furnace chamber to begin the nitridation process.
Native oxides may still form during the above conventional manufacturing process because oxygen cannot be completely removed from the load lock or inside the furnace chamber because furnace door must be opened so that the wafers may be placed inside and removed from the furnace. Therefore, despite an attempt to purge oxygen from the load lock area with nitrogen gas, oxidation of the wafers at load lock still occurs both during the transfer to boat and the boat up. Furthermore, oxidation not only occurs inside the furnace chamber during boat up but at an accelerated rate due to the elevated temperature inside the furnace chamber.
The same undesired oxidation process also occurs in a chemical-vapor deposition (xe2x80x9cCVDxe2x80x9d) process in which silicon nitride is deposited.
Accordingly, the present invention is directed to a method for preventing native oxide growth during the formation of a silicon nitride layer that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structures and methods particularly pointed out in the written description and claims thereof.
To achieve these and other advantages, and in accordance with the purpose of the invention as embodied and broadly described, there is provided a method for forming a layer of silicon nitride that includes providing at least one silicon wafer in a first chamber with ammonia gas, wherein the first chamber is substantially enclosed, and the at least one silicon wafer reacts with the ammonia gas to form a first layer of silicon nitride on the at least one silicon wafer, providing a second chamber with the ammonia gas, moving the at least one silicon wafer into the second chamber, and forming a second layer of silicon nitride on the silicon wafer.
In one aspect of the invention, the step of moving the at least one silicon wafer into the second chamber includes moving the at least one silicon wafer into a chemical vapor deposition chamber.
In another aspect of the invention, the step of forming a second layer of silicon nitride includes depositing silicon nitride on the at least one silicon wafer.
In still another aspect of the invention, the step of forming a second layer of silicon nitride includes growing a layer of silicon nitride on the at least one silicon wafer.
In yet another aspect of the invention, the method further includes sealing the first chamber with nitrogen gas at a pressure greater than one atmosphere to prevent the ammonia gas from escaping from the first chamber.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.